Today, there are a wide range of intravascular prostheses on the market for use in the treatment of aneurysms, stenosis, and other vascular disease. Balloon-expandable and self-expanding stents are well known for restoring patency in a stenosed vessel, e.g., after an angioplasty procedure, and the use of coils and stents are known techniques for treating aneurysms.
Previously-known self-expanding stents generally are retained in a contracted delivery configuration using an outer sheath or a release wire, then self-expand when the sheath or release wire is retracted. Slotted-tube stent designs typically have relatively thick struts in folding cell patterns in order to obtain a sufficient strength when deployed. In the absence of separate, costly marker bands, slotted tube stents also require thick struts for the stent to be radiopaque under x-ray imaging. Accordingly, such stents may not be suitable for use in smaller vessels, such as cerebral vessels and coronary arteries where device profile is more critical. The thick struts have also been know to cause disruptive local turbulence of blood flow in an artery, which results in negative cellular response including restenosis. An alternate stent made in a helical pattern may have a thin strut and a high surface area density for radiopacity, while also having desired strength, but may have larger than desired profile due to overlapping in the wound down state. It is desirable to have a thin strut stent design that minimizes overall delivery profile and optimizes delivery flexibility, strength, and radiopacity.
Stents also face challenges with limited vessel area coverage, resulting in poor scaffolding and subsequent restenosis. The limited vessel coverage also prohibits the use of current stents in the treatment of aneurysms. While covered stent grafts may be used, they are often restricted from use around vessels with side branches or bifurcations.
Further, slotted tube stents have suffered from poor in vivo performance in vasculature with challenging biomechanics such as the superficial femoral artery (SFA). When used in the treatment of the SFA, stents may have insufficient radial force, resulting in poor patency and restenosis. Additionally, repetitive lateral loading and unloading of a stent in the SFA have been known to cause fatigue-induced strut failure, which may contribute to restenosis and subsequent vessel narrowing and/or occlusion. Therefore, stents are desired to be configured to provide a high radial strength while also providing adequate flexibility. However, providing additional radial strength has generally resulted in a reduction in the flexibility of the stent.
Various techniques may be utilized to characterize those attributes of stents such as measuring a displacement response for a given force or determining elastic modulus. For example, radial strength may be characterized by determining the amount of force required to radially compress a stent a given distance and determining the stiffness (referred to herein as “Krad”). Alternatively, radial strength may be characterized by determining an amount of force applied linearly by opposing plates to compress a stent a given distance and determining the stiffness (referred to herein as “Kfp”). The flexibility of a stent may be characterized by measuring the amount of force required to cause a given length of axial extension and determining the stiffness (referred to herein as “Kax”). Alternatively, the flexibility may be characterized by measuring the lateral deflection of a stent in response to a lateral force applied to a free end of the stent to determine stiffness (referred to herein as “Klat”).
A ratio of the stiffness characterizations of radial strength to flexibility may be used to provide a comparison of stents having different structures. For example, it has been found that a selected sample of currently commercially available stents generally possess Krad/Kax ratios in the range of 5-10, Krad/Klat ratios in the range of 148-184, Kfp/Kax ratios in the range of 1-6 and Kfp/Klat ratios in the range of 40-113 based on test samples each having approximately a 0.236 inch diameter, 0.906 inch length and a 0.008 inch wall thickness. It would be desirable to have a structure that provides greater ratios so that a more optimum combination of radial strength and flexibility is provided.
U.S. Pat. No. 5,707,387 to Wijay describes a stent constructed from a plurality of bands, where each band is composed of a solid wire-like material formed into a closed, substantially rectangular shape. Each band is circumferentially offset from the adjacent band and adjacent bands are connected by one or more cross-tie members. This stent has several drawbacks. The rectangular cell design does not allow for longitudinal loading because the cells are not flexible. Therefore, under a longitudinal load, the apex will move out of plane and be biased into the vessel (i.e. into the vessel flow). Secondly, the stent may be susceptible to fracture with repetitive loading and unloading because of the rigid cells.
When utilizing stents in interventional procedures, it may be advantageous to deliver therapeutic agents to a vessel wall via coating on the surface of the stent. Drug eluting stents have many advantages in reducing the rates of restenosis. Typically, the drug is disposed in the matrix of a polymer coating on the struts of the stents, and then gradually released into the surrounding tissue. The uniform delivery of the drug is typically limited by the overall surface area or coverage of the stent and the proximity between struts.
In traditional slotted-tube stent designs, when deployed at the treatment diameter, large gapping between struts and low surface coverage occur as a result of the expansion from the collapsed state, typically resulting in 25% and lower metal coverage. Drug delivery from the stent struts is therefore limited by the low surface coverage of the stent.
Spiraling mesh ribbon stent designs provide certain benefits over slotted-tube stent designs for scaffolding, surface area coverage, and radial stiffness. However, such stent designs also face limitations. First, some spiralling mesh ribbon stent designs, when retained in the delivery system, face challenges with device profile as a result of multiple wound layers. While having a helical style stent not overlapped in the wound-down state reduces profile, it results in a substantial length change with the stent substantially foreshortening upon deployment. Therefore, it is advantageous to design a lower profile stent by reducing the number of layers resulting in the wrapped-for-delivery stent.
In addition, during stent deployment using an outer sheath retraction mechanism, spiraling mesh ribbon designs are susceptible to axially collapsing of the wrapped stent. A design with thin stent wall thickness may result in axially adjacent layers slipping over each other. In the constrained state, it is critical for the stent to have the axial compressive stiffness to withstand the load applied by the outer sheath during deployments. It is also important that layers remain well-aligned during stent deployment. Proper layer-layer alignment helps to resist layers shifting out of plane and the resulting disruption to deployment mechanics or high deployment forces.
Another challenge to spiraling mesh ribbon designs is achieving a successful stent deployment with the stent fully apposed to the vessel wall. One mechanism of deployment is for current helical stent platforms to unwind from the constrained state inside a catheter, either by rotating the delivery system or by allowing the stent to freely open. The unwinding sequence may be disrupted by vessel narrowings or the catching of layers, resulting in stent narrowings, which may be prohibitive to post-stent balloon dilatation. In order to successfully balloon a helical winding, it is necessary to unwind the helical section and transpose the material either distally or proximally along the stent. This can lead to a disruption in the stent apposition in these areas or be unsuccessful in creating permanent apposition. It is desirable to create a stent with less unwinding required and a counter-wind to absorb the effect of ballooning.
Deployment of spiraling mesh ribbon designs may also be challenging with respect to deployment force. Using the conventional sheath-withdrawal mechanism for self-expanding stent deployment, the outward acting forces of the multiple layers of the constrained stents may result in embedding effects inside the outer sheath, particularly after sterilization or storage-time related creep. Therefore, it would be beneficial to incorporate stent design features to lower the outward acting force of the stent when constrained in the catheter sheath and design elements to minimize stent embedding into the sheath.
Processing of mesh ribbon designs inside a catheter poses another challenge. Mesh ribbon designs typically need to be wound in order to diametrically reduce the profile of the device, thus making conventional crimping processes challenging to incorporate. Thus, it is desirable to integrate features in the design that facilitate the ability to process the prosthesis inside a small-profile catheter.
The flexibility of some spiraling mesh-ribbon stent designs is deficient as well. In one example, U.S. Pat. No. 5,603,722 to Phan et al. describes a stent formed of expandable strip-like segments. The strip-like segments are joined along side regions in a ladder-like fashion along offsetting side regions. A shortcoming of such a stent is that the junctions between adjacent segments are not provided with a means of addressing longitudinal loading. In another example, U.S. Pat. No. 5,632,771 to Boatman et al., the helical stent design has a solid backbone down the length, prohibiting the stent from elongating or foreshortening. In both cases, the result of high axial stiffness is that the stent is susceptible to strut fracture.
In view of the drawbacks of previously known devices, it would be desirable to provide an implantable vascular prosthesis with thin struts, optimal delivery flexibility and profile, robust opening dynamics amenable to post-stent balloon dilatation, high radial stiffness to flexibility ratio, longitudinal flexibility, good radiopacity, and a high surface area for both drug delivery and scaffolding, while allowing blood flow to side branch vessels.